On the assumption that you're running Windows, go to the calculator. (Start button, programs, accessories, calculator) Set it to "scientific" and enter the decimal number you want changed to hex. Switch it to hex and you will have the result. Voila!

Because no one has explicitly said this yet (although garublador came close)...

Hex works exactly the same way every number base works. Every digit in a number represents the number of times a certain power of the base is in the number. Starting from the decimal and moving left, the places are base^0, base^1, base^2, base^3, etc.